This invention relates to a transfer valve apparatus for activating a backup metering valve through which the flow of fuel to an aircraft engine is scheduled as a function of an operator input and the discharge pressure produced by a compressor in the engine when a malfunction occurs in a primary metering valve.
In a known fuel control system disclosed in U.S. Pat. No. 4,033,112, a backup metering valve responds to a manual input for supplying fuel to an engine in the event that a malfunction occurs in the primary metering valve. During manual operation, surging of the engine could occur which would cause the compressor of the engine in the aircraft to stall if the rate of fuel flow increases too rapidly.
It is well known that as the speed of a compressor increases, the weight flow of air through the compressor must also increase. However, there is a limit to the rate at which airflow can be increased. The compressor which pumps air by means of blades, is dependent on smooth aerodynamic airflow in order to function properly. If the rotational speed of the compressor is accelerated faster than the airflow can accelerate, the smooth airflow required is interrupted and turbulence occurs. This turbulence reduces the airflow to the combustion chamber resulting in too great a volume of fuel in the combustion chamber. Thus, the fuel flow to the engine and the airflow must be matched with each other in order for the engine to operate effectively in meeting an input request of an operator.
In copending U.S. patent application Ser. No. 137,659, filed Apr. 7, 1980, it was discovered that the operation of a manually operated backup valve could be modified by a compressor pressure signal and thereby match the fuel flow with the airflow during acceleration to prevent surging. However, in this fuel control it is necessary to move a power lever to a separate operating range on the power lever in order to manually schedule fuel to the engine. Unfortunately, when the power level is moved out of the backup operating range to the cutoff position, the transfer valve is deactivated and fuel scheduled through the electrically operated metering valve at its failed position, causes the engine to surge. If an engine surge occurs during a landing, the control needed to safely land an aircraft is jeopardized.